Gemara

Question: What is the source that two relatives cannot testify together (about someone else)?

Answer #1 (Rami bar Chama): Reasoning teaches this. Witnesses are not made Zomemim unless both are Huzmu. If relatives could testify about someone else, they could be killed due to each other's testimony!

Objection (Rava - Mishnah): If David testified about three years of Chazakah, and three brothers testified, each about one of the three years, (this is testimony of Chazakah, for) testimony of each year is separate. It is like one testimony for Hazamah (they are exempt unless all of them are Huzmu).

Question: If they will be Huzmu, the brothers will pay due to each other's testimony!

Answer: You must say that they pay due to the Hazamah, not due to each other's testimony

Likewise, if relatives testified together and were Huzmu, they are killed due to the Hazamah, not due to each other's testimony!

Answer #2 (Rava): Rather, the Torah could have said 'Ben Al Avos' or 'Hem Al Avos.' Instead, it says "u'Vanim" to teach that Banim (i.e. relatives) cannot testify with each other.

Gitin 17a (Mishnah): If a Get was written during the day and signed at night, it is Pasul;

R. Shimon is Machshir.

86a (Mishnah): If witnesses signed a Get, but there is no date on it, it is Pasul, but if she remarried, the children are Kosher (not Mamzerim).

Bava Basra 171a (Beraisa - R. Yehudah): If the date on a document is Yom Kipur or Shabbos, we assume that it was postdated. It is valid;

R. Yosi says, it is invalid.

(R. Pedas): If the date corresponded to Yom Kipur or Shabbos, all agree that it is valid (it is clear that it was postdated). They argue about a regular postdated document. R. Yosi disqualifies it, lest it be used to swindle.

Rishonim

Rif (Bava Kama 28b): Hazamah of Edei Chasimah (witnesses who signed on a document) is possible only if they say in Beis Din that they signed it on the day written.

Rif and Rosh (Sanhedrin 8b and 3:32): The Yerushalmi disqualifies witnesses related to each other or to the judges due to Hazamah (lest they be killed due to each other).

Ran (Kesuvos 12b DH Kivan): If the Pesul is due to Hazamah, relatives should be Kosher for documents, in which Hazamah does not apply!

Ran (Gitin 46b DH v'Get): Some say that a predated Get is like one without a date. Some say that it is worse, lest she fraudulently collect Peros from before the divorce. According to Rashi, this is not a concern. Still, perhaps she must leave, for also predated documents are Pesulim.

Rambam (Hilchos Edus 19:3): Hazamah of Edei Chasimah is possible only if they say in Beis Din that they signed it on the day written.

Rambam (Hilchos Malveh 17:9): If a document does not say where it was written, or it has no date, it is Kosher. In monetary cases we are not concerned for Drishah v'Chakirah, due to Ne'ilas Delet (lest people be deterred from lending). Therefore, postdated documents are Kosher, even though Iy Efshar Lehazimah.

Rambam (Hilchos Gerushin 1:25): If a Get has witnesses but no date, or it was predated or postdated, or it was written in Yerushalayim and they mistakenly wrote Lud, it is Pasul. It is Kosher only if it was signed on the date and in the place it was written.

Rebuttal (Ra'avad): The Rambam erred about a postdated Get. It permits remarrying once it was given, even before the date, if it is in Beis Din or he gave it in front of witnesses and did not mention that it is postdated, for her husband cannot ruin her. I say that it is as if he stipulated to eat the Peros and not pay a Kesuvah until the date written.

Kesef Mishneh: The Rambam holds that the date on a Get is only mid'Rabanan, therefore, it is Kosher mid'Oraisa even if the date is wrong. The Rosh (brought below) holds that it is Batel. Some say that a postdated Get is Kosher.

Migdal Oz: A postdated Get cannot be Kosher, for the testimony is Iy Efshar Lehazimah, so it is invalid. The only exception is a postdated loan document. It is Kosher, due to Ne'ilas Delet (lest lenders be deterred). Other documents are Pesulim, especially Gitin, for one deviated from the rules Chachamim established for Gitin. A Tosefta is Machshir, but the Gemara does not bring it. Therefore, one may not rely on the Tosefta.

Pnei Yehoshua (Gitin 36a DH Mipnei): Testimony in a document is Iy Efshar Lehazimah, unless it says that it was written on the date. It is not our custom to do so. If all Gitin were mid'Rabanan, the Gemara would have said so! Rather, the Pesul of Iy Efshar Lehazimah is not due to concern lest it is false. Rather, it is a Gezeras ha'Kasuv. Therefore, it is only where full testimony is required, e.g. capital cases and fines. Where admission or Kesav Yado helps, Edei Chasimah are no worse than Kesav Yado! A Get is not full testimony. It merely proves that the husband authorized it.

Nesivos ha'Mishpat (28:Bi'urim 7): This testimony need not be said in Beis Din, therefore, we do not need Efshar Lehazimah. The Edei Chasimah do not testify that he gave it to her; Anan Sahadi (we can testify to this). Hazamah does not apply to Anan Sahadi.

Teshuvas Rosh (45:6): A case occurred in which Levi divorced his wife. David was Mekadesh her, and then they found that her Get was predated. She does not need a Get from David. We are not concerned lest people think that his Kidushin took effect, for the Pesul of the Get became known. It was never a Get; she may return to Levi.

Nimukei Yosef (Sanhedrin Reish 10b): Why are postdated documents Kosher? This shows that we do not require Drishah v'Chakirah, nor testimony she'Efshar Lehazimah. The Torah requires it, like for capital cases. Chachamim enacted not to require it, due to Ne'ilas Delet. This enactment also allows a Get without a date. Since it is common, it is like admissions and loans. Chachamim have the power to do so, for everyone who is Mekadesh does so according to the whim of Chachamim. They enacted to uproot the Kidushin of one who divorced with such a Get. Also a Shtar Kidushin without a date works due to the enactment.

Hagahos Ashri (Kesuvos 13:20): If a document does not say where it was written, or it has no date, it is Kosher. In monetary cases we are not concerned whether or not the testimony can be Huzam.

Tosfos Rid (Gitin 80a DH umi'Shum): We write the place where a Get or document was written, so it will be Efshar Lehazimah. If not, it would be invalid.

Poskim

Shulchan Aruch (CM 43:14): If a document is dated 'Shabbos' or 'the 10th of Tishrei', it is postdated and it is valid. We are not concerned lest it is predated, and it was really written on Sunday or Tishrei 11.

Nimukei Yosef (Sanhedrin 10a DH Gemara, brought in Beis Yosef DH Kosav Rabeinu): It is unlikely that one would postdate a document and give a date in Shemitah, for it looks suspicious. People usually refrain from lending in Shemitah, lest the loan be cancelled. Even so, we assume that this occurred and Machshir the document, for loans are not cancelled until the end of Shemitah. If it was dated for the end of Shemitah and the witnesses were Huzam, it is Pasul. We would not say that it was postdated.

Shulchan Aruch (EH 127:1): If one was divorced with a Get without a date, she may not remarry. If she remarried, she need not leave.

Shulchan Aruch (2): If the Get is predated, or it was written during the day and signed at night, it is Pasul.

Rema: Even though it is Pasul, if someone else was Mekadesh her, she needs a Get from him and may not return to her first husband.